The present invention relates generally to semiconductor devices and, more particularly, to a graphene based electrically tunable nanoconstriction device.
Quantum dot (QD) technology, in particular the double-dot spin qubit scheme, has been a serious contender among solid state qubits because of its compactness and scalability. See for example “Coherent manipulation of coupled electron spins in semiconductor quantum dots”, J. R. Petta, A. C. Johnson, J. M. Taylor, E. A. Laird, A. Yacoby, M. D. Lukin, C. M. Marcus, M. P. Hanson, A. C. Gossard, Science 309, 2180 (2005). Current QD devices have predominantly been built using III-V materials, such as the GaAs/AlGaAs heterostructure. The intrinsic noise generated by the nuclear spin fluctuations in these materials severely limits the number of quantum-gate operations before decoherence sets in, posing a grave challenge for implementing quantum error correction. See for example “Dephasing time of GaAs electron-spin qubits coupled to a nuclear bath exceeding 200 μs”, H. Bluhm, S. Foletti, I. Neder, M. Rudner, D. Mahalu, V. Umansky, A. Yacoby, Nature Physics 7, 109 (2011).
Graphene, as a zero nuclear-spin system, can easily bypass the bottleneck stated above, giving graphene QDs a clear advantage over the 2DEG QDs in III-V materials. See “Spin qubits in graphene quantum dots”, Björn Trauzettel, Denis V. Bulaev, Daniel Loss, Guido Burkard, Nature Physics 3, 192 (2007). The most crucial step in building a graphene QD is the fabrication of nanoconstrictions for control of electron tunneling on and off the dot and between dots within a qubit. It is therefore essential that NO strong impurity resonance state sits in the constriction region to trap electrons.
Despite the clear research direction and countless efforts devoted to reap the benefits of graphene for quantum computation, graphene QD qubits are still yet to exist. Besides, the only report on graphene quantum point contact has been a one-shot suspended device with very limited practical value, since the constriction size is neither controllable nor tunable. See for example “Quantized conductance of a suspended graphene nanoconstriction”, Nikolaos Tombros, Alina Veligura, Juliane Junesch, Marcos H. D. Guimaraes, Ivan J. Vera-Marun, Harry T. Jonkman, Bart J. van Wees, Nature Physics 7, 697 (2011).
The root cause of this stalled progress lies in the lack of band-gap in graphene. Because of its zero band-gap, electrostatic gating for charge depletion and charge confinement is extremely difficult. Nearly all efforts on making graphene quantum devices have thus relied on defining the constriction and quantum-dot geometry using reactive-ion etching (RIE) instead of tunable electrostatic gating. See for example “Tunable Coulomb blockade in nanostructured graphene”, C. Stampfer, J. Güttinger, F. Molitor, D. Graf, T. Ihn, and K. Ensslin, Appl. Phys. Lett. 92, 012102 (2008) and “Chaotic Dirac billiard in graphene quantum dots”, L. A. Ponomarenko, F. Schendin, M. I. Katsnelson, R. Yang, E. W. Hill, K. S. Novoselov, and A. K. Geim, Science 320, 356 (2008). This etching-based geometrical confinement approach incurs severe damage to the edge of the already narrow graphene nanoconstrictions (<50 nm), causing inevitable localized resonant states and rendering the graphene dots useless for qubit applications. See for example “Quantum Dot Behavior in Graphene Nanoconstrictions” K. Todd, H. T. Chou, S. Amasha, D. Goldhaber-Gordon, Nano Lett. 9, 416 (2009) and “Imaging localized states in graphene nanosctructures” S. Schnez, J. Guttinger, M. Huefner, C. stampfer, K. Ensslin, T. Ihn, Phys. Rev. B 82, 165445 (2010).